Design Pattern _ Singleton mode

Contains only a special class called a singleton class. The singleton mode guarantees that there is only one object for a class in the system.

Advantages:

1 provides controlled access to a unique instance.

2 Save memory, only one instance of the class exists in system memory .

3 to Prevent logic errors , such as generating serial numbers, two instances may produce two identical serial numbers.

4 Allow variable number of instances.

Disadvantages:

1 time overhead: Each request reference checks for the existence of an instance of the class and still requires time overhead to resolve by static initialization.

2 Development Confusion: developers need to remember that the Singleton class cannot be created with the New keyword.

3 Life cycle: memory-managed languages such as Java,. Net), only a singleton class can cause an instance to be deallocated because it contains a private reference to the instance, a language that can manually free memory (such as C + +), an object instance can be deleted, and a floating reference is easy to appear.

4 easy to extend: The singleton mode has no abstraction layer and is not easy to expand.

5 Negative issues: Save resources and set the database connection pool object to a singleton class, causing too many programs that share connection pool objects to overflow the connection pool .

6 Object loss: The instantiated object is not exploited for a long time , and in memory-managed languages, it is reclaimed by the system, causing the object state to be lost .

Realize:

1 private, static, member variable.

2 private construction methods, which cannot be created by the new keyword.

3 The private cloning method, so that the instance cannot be copied or cloned.

4 static initialization, the getinstance () method is static, and the time overhead is reduced.

PHP instance

Class Singleton{private Static $_instance = null;//private static variable __construct () {//Private constructor method, external cannot get instance of class by New//:} Private Function __clone () {}//-clone method, cannot be copied or cloned public static function getinstance () {//static method gets instance, reduces time consumption if (! ( Self::$_instance instanceof self) {self::_instance = new Singleton ();} return self::_instance;}}

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From Arcticfox

Design Pattern _ Singleton mode